https://leetcode.cn/problems/que-shi-de-shu-zi-lcof/description/
class Solution {
public:
    int takeAttendance(vector<int>& nums) {
        int left=0,right=nums.size()-1;
        while(left<right)
        {
            int mid=left+(right-left)/2;
            if(nums[mid]==mid)
                left=mid+1;
            else
                right=mid;
        }
        return left==nums[left]?left+1:left;
    }
};